Neuromuscular junctions in newborn rats differ from adult junctions in several ways. First, the postsynaptic membrane is smooth in neonates but has extensive secondary folds in adults. Second, subneural acetylcholine receptors (AChR's) have a longer mean channel open time and a lower conductance in neonates than in adults. Finally, newborn endplates are multiply innervated but mature junctions are singly innervated. Rat soleus muscles develop postsynaptic folds, convert AChR gating properties, and eliminate excess synaptic inputs during the first three weeks after birth. This project focuses on the two postsynaptic events, particularly the shift in AChR channel properties. The single-channel recording technique will be used to characterize more fully the ways in which the gating properties of neonatal and adult AChR's differ. The mechanism behind these changes will be explored in several ways. First, an in vitro system will be used to see if phosphorylation of AChR's affects their gating properties. Second, a particular myasthenic antiserum will be tested for its ability to specifically bind to and label neonatoal-type AChR's. Third, the role of active cellular processes and of certain enzymes in mediating channel conversion will be tested by treating muscles with inhibitors. Finally, the role of the nerve in regulating postsynaptic maturation will be examined. When soleus muscles are denervated shortly after birth, both channel conversion and the formation of postsynaptic folds are inhibited. To determine if this reflects loss of a trophic substance or lack of muscle activity, we will see if direct, chronic electrical stimulation of muscles can substitute for innervation in promoting endplate maturation. The results should lead to a better understanding of synapse maturation, of AChR gating properties, and of the interactions between nerve and muscle during development.